Owing to the increasing global demand for electric energy and the limited reserves of coal, oil and gas, which in addition liberate the greenhouse gas CO2 when they are burnt, the generation of electric power from sunlight has attracted increased interest in recent years.
EP-A 0 333 641 describes a photoelectrochemical cell which comprises a nanoporous metal oxide semiconductor, i.e. a semiconductor which has an extremely roughened surface and thus an increased surface area. The charge transport between semiconductor/chromophore layer and counterelectrode in this cell occurs via an electrolyte solution. Although good results are achieved using such cells, the property profile of such a device is still capable of significant improvement.
EP-A 0 718 858 discloses such a cell having a liquid crystal charge transport material in place of an electrolyte. However, the apparent quantum yields achieved are still in need of improvement.
It has now surprisingly been found that certain derivatives of spirobifluorene are very suitable as charge transport material for photovoltaic cells.
Some structurally different spirobifluorene derivatives are described, for example, in U.S. Pat. No. 5,026,894, J. M. Tour et al. J. Am. Chem. Soc. 112 (1990) 5662 and J. M. Tour et al. Polym. Prepr. (1990) 408 as coupling elements for polymeric, organic semiconductors and are proposed as materials for molecular electronics.
EP-A 0 676 461 describes the use of spiro compounds of the following formula, 
where
K1 and K2 are, independently of one another, conjugated systems, in electroluminescence devices.
Use in photovoltaic cells cannot be deduced therefrom.
The invention accordingly provides spiro compounds of the formula (I) 
where the symbols have the following meanings:
K1, L, M, N1, R1, R2, R3, R4 are identical or different and are each
a) hydrogen, xe2x80x94NO2, xe2x80x94CN, xe2x80x94F or xe2x80x94Cl,
b) a straight-chain or branched alkyl radical having from 1 to 20 carbon atoms, where
b1) one or more nonadjacent CH2 groups can be replaced by xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94COxe2x80x94Oxe2x80x94,xe2x80x94Oxe2x80x94COxe2x80x94, xe2x80x94Oxe2x80x94COxe2x80x94Oxe2x80x94, NR5 or xe2x80x94Si(CH3)2xe2x80x94 and/or
b2) one or more CH2 groups can be replaced by xe2x80x94CHxe2x95x90CHxe2x80x94, xe2x80x94Cxe2x89xa1Cxe2x80x94, 1,4-phenylene, 1,4-cyclohexylene or 1,3-cyclopentylene and/or
b3) one or more H atoms can be replaced by F and/or Cl, and/or
c) one of the following groups: 
d) one of the following groups: 
with the proviso that at least one, preferably at least two, of the radicals K1, L, M, N1, R1, R2, R3, R4R is one of the groups listed under c);
X, Y1 are in each case identical or different and are xe2x95x90CR7xe2x80x94 or xe2x95x90Nxe2x80x94;
Z is xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94NR5xe2x80x94, xe2x80x94CRRxe2x80x94, xe2x80x94CRxe2x95x90CRxe2x80x94 or xe2x80x94CRxe2x95x90Nxe2x80x94;
R5, R6 are in each case identical or different and are each
a) hydrogen,
b) a straight-chain or branched alkyl radical having from 1 to 20 carbon atoms, where
b1) one or more nonadjacent CH2 groups which are not bound to nitrogen can be replaced by xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94COxe2x80x94Oxe2x80x94, xe2x80x94Oxe2x80x94COxe2x80x94, xe2x80x94Oxe2x80x94COxe2x80x94Oxe2x80x94 or xe2x80x94Si(CH3)2xe2x80x94 and/or
b2) one or more CH2 groups can be replaced by xe2x80x94CHxe2x95x90CHxe2x80x94, xe2x80x94Cxe2x89xa1Cxe2x80x94, cyclopropane-1,2-diyl, 1,4-phenylene, 1,4-cyclohexylene or 1,3-cyclopentylene and/or
b3) one or more H atoms can be replaced by F and/or Cl and/or
b4) R5 and R6 together can also form a ring;
c) phenyl, biphenyl, 1-naphthyl, 2-naphthyl, 2-thienyl, 2-furanyl;
R7 , R8, R9, R10, R11, R12 are identical or different and are each
a) hydrogen, xe2x80x94CN, xe2x80x94F, NO2 or xe2x80x94Cl,
b) a straight-chain or branched alkyl radical having from 1 to 20 carbon atoms, where
b1) one or more nonadjacent CH2 groups can be replaced by xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94COxe2x80x94Oxe2x80x94, xe2x80x94Oxe2x80x94COxe2x80x94, xe2x80x94Oxe2x80x94COxe2x80x94Oxe2x80x94, NR5 or xe2x80x94Si(CH3)2xe2x80x94 and/or
b2) one or more CH2 groups can be replaced by xe2x80x94CHxe2x95x90CHxe2x80x94, xe2x80x94Cxe2x89xa1Cxe2x80x94, cyclopropane-1,2-diyl, 1,4-phenylene, 1,4-cyclohexylene or 1,3-cyclopentylene and/or
b3) one or more H atoms can be replaced by F and/or Cl;
c) phenyl, biphenyl, 1-naphthyl, 2-naphthyl, 2-thienyl, 2-furanyl, xe2x80x94O-phenyl, xe2x80x94O-biphenyl, xe2x80x94O-1-naphthyl, xe2x80x94O-2-naphthyl, xe2x80x94O-2-thienyl, xe2x80x94O-2-furanyl,
m, n, p, q, r are in each case identical or different and are 0, 1, 2, 3, 4, 5 or 6 preferably 0, 1, 2, 3, 4, particularly preferably 0, 1, 2, or 3.
The compounds of the formula (II) are preferably amorphous and have high glass transition temperatures.
Preference is given to spirobifluorene derivatives of the formulae (II) a-c 
where the symbols have the following meanings:
II.a) K1xe2x95x90Lxe2x95x90Mxe2x95x90N1 and is selected from the group consisting of: 
R are identical or different and are H, alkyl, xe2x80x94O-alkyl, xe2x80x94S-alkyl, each having from 1 to 20 carbon atoms, preferably from 1 to 4 carbon atoms, phenyl, biphenyl, 1-naphthyl, 2-naphthyl, 2-thienyl, 2-furanyl, xe2x80x94O-phenyl, xe2x80x94O-biphenyl, xe2x80x94O-1-naphthyl, xe2x80x94O-2-naphthyl, xe2x80x94O-2-thienyl, xe2x80x94O-2-furanyl, CN, NR2, where xe2x80x94O-alkyl/aryl, xe2x80x94S-alkyl/aryl, CN, NO2 must not be bound to nitrogen;
n=0, 1, 2, 3, 4,
and Q, P1 are identical or different and are selected from the group consisting of: 
where the symbols and indices are as defined above,
II.b) K1xe2x95x90N1 and is selected from the group consisting of: 
and Lxe2x95x90M and is selected from the group consisting of: 
and Q, P1 are identical or different and are selected from the group consisting of: 
where the symbols and indices are as defined above;
IIc) K1xe2x95x90M and is selected from the group consisting of: 
and Mxe2x95x90N1 and is selected from the group consisting of: 
and Q, P1 are identical or different and are selected from the group consisting of: 
where the symbols and indices are as defined above.
Particular preference is given to the following compounds of the formula (II):
IIaa) K1xe2x95x90Lxe2x95x90Mxe2x95x90N1 and is selected from the group consisting of: 
where R13 is xe2x80x94Oxe2x80x94CH3, xe2x80x94Oxe2x80x94C2H5, xe2x80x94Sxe2x80x94CH3, xe2x80x94Sxe2x80x94C2H5, preferably xe2x80x94Oxe2x80x94CH3, xe2x80x94Sxe2x80x94CH3, particularly preferably xe2x80x94Oxe2x80x94CH3;
and Qxe2x95x90P1 and is selected from the group consisting of: 
where R14 is a straight-chain or branched alkyl group having from 1 to 12, preferably from 1 to 4, carbon atoms;
II.ba) K1xe2x95x90Mxe2x95x90N1xe2x95x90Qxe2x95x90P1 and is selected from the group consisting of: 
where R13 is as defined above;
II.ca) K1xe2x95x90Lxe2x95x90Mxe2x95x90N1 and is selected from the group consisting of: 
and Qxe2x95x90H and P1 is selected from the group consisting of: 
where R13, R14 are as defined above.
The preparation of the spiro compounds of the invention is carried out by methods known per se from the literature, as are described in standard works on organic synthesis, e.g. Houben-Weyl, Methoden der Organischen Chemie, Georg-Thieme-Verlag, Stuttgart and in the appropriate volumes of the series xe2x80x9cThe Chemistry of Heterocyclic Compoundsxe2x80x9d by A. Weissberger and E. C. Taylor (editors).
The preparation is carried out under reaction conditions which are known and suitable for the reactions mentioned. Use can also be made of variants which are known per se and are not mentioned in more detail here.
Compounds of the formula (I) are obtained, for example, starting from 9,9xe2x80x2-spirobifluorene, whose synthesis is described, for example, by R. G. Clarkson, M. Gomberg, J. Am. Chem. Soc. 1030, 52, 2881.
The preparation of compounds of the formula (IIa) can be carried out, for example, starting with a tetrahalogenation in the 2,2xe2x80x2,7,7xe2x80x2 positions of 9,9xe2x80x2-spirobifluorene and a subsequent substitution reaction (see, for example, U.S. Pat. No. 5,026,894) or via a tetraacetylation of the 2,2xe2x80x2,7,7xe2x80x2 positions of 9,9xe2x80x2-spirobifluorene with subsequent Cxe2x80x94C coupling after conversion of the acetyl groups into aldehyde groups or heterocycle formation after conversion of the acetyl groups into carboxylic acid groups.
The preparation of compounds of the formula (IIb) can be carried out, for example, by methods analogous to those for the formula (IIa) with the stoichiometric ratios in the reaction being selected so that the 2,2xe2x80x2 or 7,7xe2x80x2 positions are functionalized (see, for example, J. H. Weisburger, E. K. Weisburger, F. E. Ray, J. Am. Chem. Soc. 1959, 72 4253; F. K. Sutcliffe, H. M. Shahidi, D. Paterson, J. Soc. Dyers Colour 1978, 94, 306 and G. Haas, V. Prelog, Helv. Chim. Acta 1969, 52, 1202).
The preparation of compounds of the formula (IIc) can be carried out, for example, via a dibromination in the 2,2xe2x80x2 positions and subsequent diacetylation in the 7,7xe2x80x2 positions of 9,9xe2x80x2-spirobifluorene with subsequent reaction similar to that for the compounds (IIc).
Compounds of the formulae (II) in which K1, L, Q, P1xe2x95x90H and Mxe2x95x90N1 or Q, P1xe2x95x90H, K1xe2x95x90L and Mxe2x95x90N1 can be prepared, for example, by choice of appropriately substituted starting compounds in the formation of the spirobifluorene, e.g. 2,7-dibromospirobifluorene can be built up from 2,7-dibromofluorenone and 2,7-dicarbethoxy-9,9xe2x80x2-spirobifluorene by use of 2,7-dicarbethoxyfluorenone. The free 2xe2x80x2,7xe2x80x2 positions of the spirobifluorene can then be further substituted independently.
For the synthesis of the groups K1, L, M, N1, P1, R1, R2, R3, R4, reference may be made, for example, to DE-A 23 44 732, 24 50 088, 24 29 093, 25 02 904, 26 36 684, 27 01 591 and 27 52 975 for compounds having 1,4-phenylene groups; DE-A 26 41 724 for compounds having pyrimidine-2,5-diyl groups; DE-A 40 26 223 and EP-A 0 391 203 for compounds having pyridine-2,5-diyl groups; DE-A 32 31 462 for compounds having pyridazine-3,6-diyl groups; N. Miyaura, T. Yanagi and A Suzuki in Synthetic Communications 1981, 11, 513 to 519, DE-A 39 30 663; M. J. Sharp, W. Cheng, V. Snieckus, Tetrahedron Letters 1987, 28, 5093; G. W. Gray, J. Chem. Soc. Perkin Trans II 1989, 2041 and Mol Cryst. Liq. Cryst. 1989,172, 165; Mol. Cryst. Liq. Cryst. 1991, 204, 43 and 91; EP-A 0 449 015; WO 89/12039; WO 89/03821; EP-A 0 354 434 for the direct coupling of aromatics and heteroaromatics.
The preparation of disubstituted pyridines, disubstituted pyrazines, disubstituted pyrimidines and disubstituted pyridazines is described, for example, in the appropriate volumes of the series xe2x80x9cThe Chemistry of Heterocyclic Compoundsxe2x80x9d by A. Weissberger and E. C. Taylor (editors).
Amino compounds of the formula (I) can be built up by means of variants of the Ullmann reaction (J. March, Adv. Org. Chem., 4th edition, p. 665, John Wiley and Sons, New York 1992), as is described, for example, in Chem. Lett. 1989, p. 1145; Mol. Cryst. Liq. Cryst. 1994, 242, 127 and particularly in J. Salbeck et al., 213th ACS National Meeting, San Francisco 1997, Book of Abstracts p. 199. A further possibility is a process known from U.S. Pat. No. 5,576,460. Preference is given to preparing such compounds by a process disclosed in the German patent application 19738860.4 having the title xe2x80x9cVerfahren zur Herstellung von Aryloligoaminenxe2x80x9d. This application is expressly incorporated by reference into the present description.
The novel spiro compounds of the formula (I) are suitable as charge transport materials, preferably for photovoltaic cells.
The invention therefore also provides for the use of spiro compounds of the formula (I) as charge transport material, in particular for photovoltaic cells.
The invention further provides a photovoltaic cell having a charge transport layer which comprises, preferably consists of, one or more, preferably one, Spiro compound of the formula (I).